This invention relates to light-emitting semiconductor devices such as what is called the light-emitting diode in common parlance, particularly to those employing nitrides or nitride-based compounds as semiconductors, and more particularly to an active layer of improved quantum well structure included in such light-emitting devices. The invention also pertains to a method of making such light-emitting semiconductor devices.
Japanese Unexamined Patent Publication Nos. 8-264832 and 2001-313421 are hereby cited as dealing with high-intensity light-emitting nitride semiconductor devices. For production of light ranging from ultraviolet to green, these known devices have their active region made from the class of gallium nitrides that are generally defined as InxAlyGa1-x-yN where x and y are both equal to or greater than zero and equal to or less than one and where the sum of x and y is equal to or greater than zero and equal to or less than one.
As heretofore constructed, light-emitting semiconductor devices have a substrate of sapphire, silicon carbide, or silicon on which there are grown gallium nitride semiconductor layers. In the case of a sapphire substrate, for instance, there is formed thereon a lamination of GaN buffer layers, a silicon-doped n-type GaN contact layer, an n-type AlGaN semiconductor layer, an active region of quantum well structure, a p-type AlGaN semiconductor layer, and a magnesium-doped p-type GaN contact layer, in that order from immediately over the substrate farther away therefrom.
Two different types of quantum well structures are known in the art: multiquantum well and single quantum well. The active region of both types includes a well layer or layers and barrier layers. Both well layers and barrier layers are made from InGaN for example, but the well layers contain indium in a greater proportion than do the barrier layers. The well layers are usually grown at temperatures not exceeding 800° C. with a view both to prevention of the decomposition of indium and to higher crystallinity. The usual practice in the art has been to grow the barrier layers approximately at the same temperature as the well layers. The resulting barrier layers have not necessarily been satisfactory in crystallinity, bringing about some difficulties and inconveniences set forth hereinbelow.
The barrier layers of poor crystallinity are, first of all, incapable of sufficiently restricting the vaporization of indium from the well layers, thereby allowing the well layers to deteriorate in crystallinity, too. Another inconvenience is the mutual diffusions of indium from the well layers and gallium from the barrier layers, beyond the interfaces between the well and barrier layers. The well layers have thus been easy to become irregular in virtual thickness, resulting in fluctuations in the wavelength of the light emitted. Furthermore, during the subsequent growth of the noted p-type GaN layer on the active region at 1100° C. or so, the barrier layers of poor crystallinity have further promoted both of indium evaporation from the well layers and the mutual diffusions of indium and gallium beyond the interfaces between the well and barrier layers. The diffusion of impurities such as magnesium from p-type GaN layer to active region, with the consequent deterioration of the crystallinity of the active region, has also had to be feared.